Circuit of a loop sensor based simple security alarm is described here. The sensor loop is nothing but a short length of thin enamelled copper wire, which bridges two input points of the electronic alarm circuit. When the loop is opened the alarm circuit fires an active electric/electronic hooter to raise an ear-splitting audio alert signal. The circuit requires 12 volt dc supply for proper working.
When the loop is closed, anode terminal of diode D1 is at ground level and transistor T1 is off. When the loop opens, capacitor C1 is quickly charged via resistor R1 and diode D1, whereupon mosfet T1 comes on so that the hooter (BZ1) is switched on. If the loop is closed again, initial condition is maintained by grounding the anode of D1 and this stops the charging of C1. However, C1 is disharged fairly slowly via R2, so that T1 is not switched off immediately! This ensures that the alarm remains active for a little more time and then goes out slowly. This timeout can be changed by varying the value of R1,R2 and C1. Mosfet T1 may be one of many types of popular n-channel power mosfet, but it should be able to handle the selected 12V hooter of your choice.